Doped, metal oxide-based chemical sensors

ABSTRACT

The present invention generally relates to doped, metal oxide-based sensors, wherein the doped-metal oxide is a monolayer, and platforms and integrated chemical sensors incorporating the same, methods of making the same, and methods of using the same.

FIELD OF THE INVENTION

The present invention generally relates to doped, metal oxide-basedsensors, wherein the doped-metal oxide is a monolayer, and platforms andintegrated chemical sensors incorporating the same, methods of makingthe same, and methods of using the same.

BACKGROUND OF THE INVENTION

Chemiresistors (conductometric sensors) are traditionally used asbuilding blocks for integrated chemical sensors (artificial olfactorysystems, electronic noses). A chemiresistor is a device whose electricalresistance is modulated by molecular adsorption on its surface. Thechanges in resistance are proportional to the partial vapor pressure.Hence a chemiresistor converts the concentration of chemicals in theambient air into a corresponding measurable electrical signal. Achemiresistor is constructed from a vapor-sensitive material placedbetween conducting leads. One of the best chemically sensitive materialsever discovered is nanoscale tin oxide (SnO₂). The sensing mechanism ofmetal oxides is primarily based on the activation of atmospheric oxygenon the semiconductor surface. Consequently, catalytic reactions ofgaseous species with oxygen sites on the surface induce charge transferfrom the surface to the bulk, i.e., subsurface, which changes theelectrical resistance of the device.

Most conventional chemical sensors are based on recognition ofparticular analytes, e.g., methane, carbon monoxide, hydrogen sulfide,etc. For complex mixtures, however, this approach is not the mostreliable, since it causes frequent false alarms due to cross-sensitivityof sensors to different analytes. The advantage of using integratedsensory systems (electronic noses) is in their ability to learn thechemical signatures of interest, similar to training of canines. Unlikemany other analytical techniques, an integrated sensor does not try toseparate all the chemical components within a sample, but it perceives asample as a whole, creating a global fingerprint. For example, the smellthat emanates from coffee has hundreds of different chemical components,but our biological olfactory system (and the integrated sensor) simplyidentifies the total chemical composition as coffee. In an integratedsensor, the headspace from a sample (i.e., the gases emanating from asample) is delivered to an array of chemical sensors. As each sensor isdifferent in some way (usually broadly tuned to a different chemicalgroup), each sensor's response to a sample is different. These responsescan then be used to form a chemical fingerprint of a sample. Theresponse is seen as a change in electrical properties (normallyresistance) of the sensor. Specialized software then identifies thesample from this fingerprint.

These sensors usually suffer more or less from cross sensitivity, i.e.,apart from their response to a particular target gas they do (to acertain extent) respond to other gases as well. For instance, a methanesensor is also responsive to propane, butane, and natural gas ingeneral. In this respect, a single output sensor cannot be sufficient,even if only one target gas is to be detected. However, a combination ofseveral gas sensors, each providing a different sensitivity spectrum, aso-called sensor array, delivers signal patterns characteristic for thegases to which the array is exposed. These signal patterns enable thedistinction between individual gases or gas ensembles. Which gases canor cannot be detected or distinguished depends on the sensor type andthe extent of the difference in selectivity between the sensors. Now,the cross sensitivity of the individual sensors, due to a lowselectivity even turns out to be an advantage. A low selectivity (in thecase of a single sensor a disadvantage for detecting a particular gas),now allows the array to respond to a wide range of gases. A combinationof several chemiresistors, each providing a different sensitivityspectrum, a so-called sensor array, delivers signal patternscharacteristic for the gases to which the array is exposed.

For more than two decades now, small and simple gas sensors, whichprovide one output signal only, have been commercially available.Typically, they are manufactured by the sol-gel method, in which metaloxide layers are deposited in the form of a viscous paste and then bakedin an inert environment, creating thick films. Metal oxide sensors fromFigaro (TGS sensors) and Henan Hanwei Electronics Co., ltd. (MQ sensors)are manufactured by this method. The first integrated sensory systemsequipped with arrays assembled from separate sensors were manufacturesin the early 90s.

Individually manufactured sensors equipped with sockets are placed inplugs on a carrier plate of several centimeters in size. There aremultiple drawbacks associated with this conventional design:

-   -   1. The sol-gel process utilized for manufacturing of individual        sensors does not provide precise control over the oxide layer        thickness. Because of that, variations from sensor to sensor in        this manufacturing process are unavoidable. As a consequence,        even if the datasheet provides a calibration curve, every sensor        manufactured by the sol-gel method requires a calibration and        verification by the consumer, using costly specially prepared        gaseous mixtures.    -   2. Individual sensors in a sensor array evolve over time. This        phenomenon is known as a long-term drift. For a conventional        integrated system, individual elements evolve differently,        causing failures of pattern recognition algorithms.    -   3. Short-term drift due to the fluctuations in the environment        also has different effects on individual elements in a        conventional integrated system and causes failures of pattern        recognition algorithms.    -   4. Frequently, individual sensors in a conventional integrated        system have variances in response time. This means that some of        them respond to analyte exposure faster than the others. Upon        exposure to analyte but before reaching a stationary state,        sensors of integrated system go through the transient phase. If        they are not well-synchronized, during the transient phase, the        conventional integrated system typically reports several false        results. Synchronization of individual elements of a        conventional integrated system is another time consuming        process, and has to be implemented for each unit after the        assembly.    -   5. If one of the sensors in a conventional integrated system        fails and needs to be replaced, the entire system will need to        undergo synchronization and calibration.    -   6. The thickness of the metal oxide layer is a key parameter        that determines sensor sensitivity. The thinner the layer—the        higher the sensitivity. Since only the thick films can be        produced by the sol-gel method, sensors formed with this method        have limited sensitivity. For most chemical compounds, the        sensitivity of a sol-gel sensor is unable to go below 1 ppm.    -   7. Sol-gel films, which are thick, have relatively long time of        recovery after exposure, which can be up to 1 min for exposure        to high concentrations.    -   8. Conventional integrated sensory systems are typically large        in size. Discrimination power of an integrated sensor depends on        the number of individual basic sensing elements with different        chemiresistive properties. However, an increase in the number of        sensors inevitably leads to an increase in size, which causes        makes a non-uniform distribution of chemicals over the sensor        array upon exposure to gaseous analyte.    -   9. As a consequence of their size, conventional integrated        systems often require sophisticated gas sampling systems,        splitting the analyte gas into identical fractions for each        sensor.    -   10. Conventional integrated sensory systems typically have high        power consumption (hundreds of watts).    -   11. Conventional integrated sensory systems typically have high        manufacturing costs, especially in the case of an advanced        sampling system. Synchronization and calibration also makes the        manufacturing process time-consuming.

In view of the above, it would be advantageous to develop newchemiresistors and integrated chemical sensors and methods of making andusing the same, which overcome at least some of the above-noteddrawbacks with conventional integrated sensory systems.

SUMMARY OF THE INVENTION

In an aspect, the present invention provides novel, doped, metaloxide-based chemical sensors.

In an aspect, the present invention provides novel, doped, metaloxide-based chemical sensor platforms.

In an aspect, the present invention provides novel, doped, metaloxide-based integrated chemical sensors.

In another aspect, the present invention provides a novel method ofmaking doped, metal oxide-based chemical sensors, platforms and/orintegrated chemical sensors.

In another aspect, the present invention provides use of novel, doped,metal oxide-based chemical sensors, platforms, and/or integratedchemical sensors.

These and other aspects, which will become apparent during the followingdetailed description, have been achieved by the inventors' discovery ofnew doped, metal oxide-based chemical sensors; doped, metal oxide-basedchemical sensor platforms; and, metal oxide-based integrated chemicalsensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a heating element (white area surrounding the green “T” inthe middle) and electrical leads (interdigitated terminals)(remainingwhite area) deposited on a wafer (green).

FIG. 2 shows a wafer (green) after trenching (black areas, etchedsilicon) and removal of the photomask. The wafer is 1.875×1.875 mm.

FIG. 3 shows an example of a “floating sensor” (which can also bereferred to as a membrane with SiO₂/Si/SiO₂ connectors).

DETAILED DESCRIPTION OF THE INVENTION

In order to overcome the typical limitations of conventional integratedsensory systems, a new highly-integrated multisensory system andmanufacturing technique was created. The newly developed manufacturingtechnique is simple, straight-forward, inexpensive, controllable, andrepeatable.

In order to miniaturize the sensor and minimize power consumption, adesign was developed that allows one to place multiple sensing elementson a single silicon chip. This “lab-on-a-chip” design utilizes an arrayof metal leads (e.g., platinum and gold), deposited on a silicon waferusing the stencil (shadow mask) method. Deposition of electrodes isfollowed by the deposition of catalytic, doped metal oxide layers. Themultisensory system is wire-bonded to a multi-pin packaging platform forfurther integration in an electronic device.

For efficient recognition of analytes, each sensor of the integratedsensory system can be tuned to a certain chemical group (selective);hence each sensor's response to a gaseous species can be different.These responses can then be used to form a chemical fingerprint of ananalyte. The discrimination power of the artificial olfactory systemcomes from the integrated signal from the entire array of sensors.Selectivity of each sensor in the array is defined as the ability topromote only the rate of desired chemical reaction and also retard theundesired reactions.

In an aspect, the present invention relates to a novel manufacturingtechnique that allows for tuning the catalytic selectivity of thesensors in the desired manner. This aspect of the invention is based onco-sputtering a metal oxide (e.g., SnO₂) with a dopant (e.g., a secondoxide such as TiO₂ or In₂O₃)(or using another technique that providesfine control of layer thickness and repeatability).

There are several physical and chemical mechanisms that can beattributed to the improvement of the chemiresistor sensitivity andselectivity upon nanoparticles deposition. The improvement is determinedby the two primary mechanisms associated with the surface doping ofoxides with metal nanoparticles.

The first mechanism is determined by the differences in catalyticactivity of nanoscale metals, which is known as the “spillover effect”.The presence of nanoparticle dopants (e.g., TiO₂ nanoparticles) lowersthe electronic work function and decreases the activation energy of thecatalytic reaction occurring on the surface of the metal oxidenanograins. The products of catalysis occurring on the metal or metaloxide nanoparticles diffuse onto the metal oxide support (e.g., SnO₂),which acts as a reagent delivery system for the metal or metal oxidenanoparticle, amplifying the chemical processes occurring at the metaloxide surface (e.g., SnO₂), and dramatically modifying the oxideelectronic behavior. The binding energy of the ionized oxygen species inthe steady state, the catalytic reaction rate upon exposure to a vaporpulse, and the charge transfer rate from the surface to the bulk of themetal oxide (e.g., SnO₂) is strongly dependent on the electronic workfunction of the nanoparticles (e.g., TiO₂) and metal oxide support(e.g., SnO₂). Hence, the same analyte will generate differentconductivity changes in chemiresistors functionalized with differentnanoparticles (e.g., TiO₂ vs. Pt or In₂O₃ vs. TiO₂).

The functionalization of chemo resistor can be obtained by two differentmethods: surface modification, when deposition of metal oxide (e.g.,SnO) is followed by deposition of modificator (e.g., TiO₂, Pt), or bulkmodification when co-sputtering or multiple consecutive depositions oftwo metal oxides can be used in order to obtain nanocomposite materialall through the volume of the sensor.

The second mechanism has a primarily physical nature, but also stronglydepends on the work function of the vapor-sensitive materials. Thedeposition of nanoparticles (e.g., TiO₂, In₂O₃, Au, Pd, or Pt) on thesurface of semiconductor layer (e.g., SnO₂ nanograins) or incorporatingthe nanoparticles (e.g., TiO₂, In₂O₃, Au, Pd, or Pt) all through thevolume of the semiconductor layer (e.g., SnO₂ nanograins) leads to theformation of a large amount of nanoscale metal-semiconductor Schottkycontacts (Schottky barriers) or semiconductor-semiconductor junctions(n-N or n-p junctions) over the metal oxide layer. The energy barrierheight of the contacts is determined by the difference between the workfunctions W of the metal and semiconductor ΔE=Wm−Ws or difference in thework function between two different types of semiconductors ΔE=Ws1−Ws2.For example, the coating of SnO2 with catalytic nanoparticles such as Pt(W=5.12-5.93 eV), Pd (W=5.22-5.6 eV), Au (W=5.1-5.47 eV), Ni(W=5.04-5.35 eV), and Cu (W=4.53-5.1 eV) leads to a formation ofSchottky contacts with different barrier heights. In theSnO2-nanoparticle chemiresistor, the external electrostatic potential isinduced on nanoparticles by the adsorbed oxygen species. Exposure of thechemiresistor to a vapor pulse temporarily removes the adsorbed oxygenand causes a drop in the electrostatic potential induced on nanoparticlecausing charge transfer from the nanoparticle to the semiconductorsupport. This mechanism of current modulation is similar to the fieldeffect transistor, where the nanoparticle acts as a gate and theadsorbed oxygen molecules work as a source of electrostatic potential.Since the depletion depth and the amount of transferred charge are bothdetermined by the value of ΔE, the physical discrimination mechanism isalso determined by the work functions of the nanoparticles and SnO₂support.

In the present invention, the catalytic reaction activation energy istuned in a desired manner. It now has been found that certainmodifications make catalytic properties of sensors highly-preferentialtoward a particular analyte. In this way, the surface is able to triggeronly the reactions with particular activation energy. The sensors of thepresent invention can be separated into five groups:

-   -   1. Sensors of gases acidic in nature: these sensors are        SnO₂-based and tuned toward acidic sensitivity by surface doping        with Cu nanoparticles forming basic oxide CuO. The basic nature        of CuO makes the interactive layer very selective to acidic        gases like H₂S and mercaptans.    -   2. Sensors of gases basic in nature: these sensors are        SnO₂-based and tuned toward basic sensitivity by surface doping        with Mo nanoparticles forming acidic oxide MoO₃. These sensors        have preferential sensitivity towards NH₃ and amines.    -   3. Sensors of oxidizing gases (e.g., O₂ and NO₂): these sensors        are SnO₂-based and tuned toward oxidizing sensitivity by surface        doping with Ni nanoparticles forming oxide NiO.    -   4. Sensors of reducing gases without well-pronounced        acidic/basic properties (e.g., CO, H₂, and CH₄): these sensors        are SnO₂-based and tuned toward reducing gases by surface doping        with nanoparticles of noble metals forming quasi-stable        clusters, e.g., Pd_(n)O_(m) and Pt_(n)O_(m).    -   5. Sensors of organic vapors (e.g., ethanol, benzene, toluene,        ethylbenzene, xylene etc.): These sensors are SnO₂-based and        tuned toward acidic sensitivity by doping with Fe nanoparticles        forming basic oxide Fe₂O₃.        Deposition of Gas-Sensitive Layers:

In an aspect of the present invention the metal oxide and dopant layersare applied by sputtering. Conventional sensors are prepared by thestandard sol-gel technique, which is based on deposition of smalldroplets of metal oxide paste. The present sputtering technique issuperior to sol-gel method for the following reasons.

The first major drawback of sol-gel method is the large layer thickness,and hence limited sensitivity. Second, the deposition of microdropletsdoes not provide a complete control over the layer geometry andthickness. As a consequence of that, variations from sensor to sensor inthis manufacturing process are unavoidable. Third, sensor response andrecovery time for thick films is much longer than the thin filmsobtainable with sputtering, especially for large concentrations ofanalytes (gases). Because of the lack of control over sol-geldeposition, sensor voltage output cannot be predicted. Therefore, eachsensor requires independent calibration using special gas mixes, whichadds manufacturing cost and time. Oxide powder, used for pastepreparation in sol-gel method, consists of micrograins, obtained bymilling of bulk metal oxide. This technology is also known as top-downapproach. Multiple studies indicate that microstructures obtained bytop-down approach have relatively low chemical reactivity andsensitivity, compared to self-assembled nanograins, obtained bybottom-up approach (e.g., sputtering).

In contrast to the sol-gel method, the present sputtering method createsan ultra-thin layer of a precisely controlled geometrical shape. Thisprovides: ultra-high sensitivity, ultra-fast response and recovery time,and elimination of variations from sensor to sensor. Sensor behaviorbecomes predicable, because the manufacturing process is fullycontrollable. Sensors formed this way can be implemented immediately andindependent calibration of every device is no longer necessary forrelatively high concentrations (50 ppm and higher). Of course, forultra-precise sub-ppm measurements, calibration is still useful.Finally, the present manufacturing technique is based on self-assemblyof metal-oxide structures of complex multi-oxide nature (SnO₂/TiO₂ orSnO₂/In₂O₃) and further possible surface functionalization with metalnanoparticles, also known as surface doping.

Integrated System Design, Manufacturing and Maintenance

The present manufacturing technique allows for fast manufacturing oflarge quantities of sensors (e.g., the simultaneous manufacturing of 624chemical sensor platforms on a single wafer). For conventional sol-gelsystems it would be equivalent to precise targeting and deposition of2496 droplets, which is extremely expensive and time consuming.

Conventional integrated sensory systems are typically large in size.Discrimination power of an integrated sensor depends on the number ofindividual basic sensing elements with different catalytic properties.However, an increase in the number of sensors inevitably leads to anincrease in size, which causes a non-uniform distribution of chemicalsover the sensor array upon exposure to gaseous analyte leading to falserecognition. In contrast to a conventional system, the disclosedhighly-integrated system provides a physical placement of all thesensors at the same point (or nearly) in space. This design assures auniform exposure of all the sensors to chemicals and, hence, an accuraterecognition and concentration measurements.

Because of the large size, conventional integrated systems requiresophisticated gas sampling systems, splitting the analyte gas intoidentical fractions for each sensor. Conventional integrated sensorysystems typically have high manufacturing costs, especially in case ofan advanced sampling system. Synchronization and calibration makes themanufacturing process time-consuming. Conventional integrated sensorysystems typically have high power consumption (hundreds of watts). Incontrast, the disclosed integrated system performs accurately even witha very primitive sampling system, can consume about tens of mV of powerand has a manufacturing cost lower than that for a simple single-gasconventional sensor.

Operational Characteristics of Integrated Systems:

Individual sensors in the array evolve over time. This phenomenon isknown as a long-term drift. For a conventional integrated system,individual elements evolve differently, causing failures of patternrecognition algorithms. Short-term drift due to the fluctuations in theenvironment also has different effect on individual elements and alsocauses instabilities in pattern recognition algorithms.

Frequently, individual sensors of a conventional integrated system havevariances in time constant, response, and recovery time. This means thatsome of them respond to exposures faster than the others. Upon exposureto analyte, before reaching the stationary state, sensors of anintegrated system go through a transient phase. If they are notwell-synchronized, during the transient phase, the integrated systemtypically reports several false results. Synchronization of individualelements of a conventional integrated system is another time consumingprocess, and has to be implemented for each unit after the assembly.

An advantage of the present invention is that all the elements of thehighly integrated array have the same dynamics for the long-term andshort-term drift. Also, the time constant, response and recovery time isthe same for all of them, meaning that the sensors are synchronized.Thanks to that the robust recognition is preserved even during thetransient response.

In an aspect, the present invention provides a novel chemical sensor,comprising:

-   -   (a) an oxidized silicon wafer, comprising: a silicon layer        sandwiched between a top (1^(st)) silicon oxide (SiO₂) layer and        a bottom (2^(nd)) SiO₂ layer, the top SiO₂ layer, comprising: a        sensor area;    -   (b) a heating element in contact with the 1^(st) SiO₂ layer and        located near at least one edge of the sensor area;    -   (c) a pair of electrical leads in contact with the 1^(st) SiO₂        layer and at least partly located on the sensor area; and,    -   (d) a doped, metal oxide layer located on the sensor area and in        contact with at least a part of the pair of electrical leads and        the 1^(st) SiO₂ layer, wherein the doped, metal oxide layers,        comprises:        -   (i) a 1^(st) metal oxide; and,        -   (ii) a 2^(nd) metal oxide.

In an aspect, the present invention provides a novel chemical sensor,comprising:

-   -   (a) an oxidized silicon membrane, comprising a silicon (Si)        layer and a silicon oxide (SiO₂) layer, wherein the SiO₂ layer        is located on top of the silicon layer and, comprises: a sensor        area;    -   (b) a heating element in contact with the SiO₂ layer and located        near at least one edge of the sensor area;    -   (c) a pair of electrical leads in contact with the SiO₂ layer        and at least partly located on the sensor area; and,    -   (d) a doped, metal oxide layer located on the sensor area and in        contact with at least a part of the pair of electrical leads and        the SiO₂ layer, wherein the doped, metal oxide layers,        comprises:        -   (i) a 1^(st) metal oxide; and,        -   (ii) a 2^(nd) metal oxide.

Membrane (sometimes referred to as a “floating” sensor) refers to aSiO₂/Si wafer that is typically formed from an oxidized silicon wafer(e.g., a wafer having SiO₂/Si/SiO₂ layers). The membrane is formed byremoving one of the SiO₂ layers (e.g., the bottom layer) and asubstantial portion of the Si layer. Typically part of the originalwafer (e.g., SiO₂/Si/SiO₂) is left to serve as connectors for themembrane (e.g., leaving the 4 corner pieces of the original wafer as the“connectors” to the membrane).

In another aspect, the present invention provides a novel chemicalsensor platform, comprising:

-   -   (a) an oxidized silicon wafer, comprising: a silicon layer        sandwiched between a top (1^(st)) silicon oxide (SiO₂) layer and        a bottom (2^(nd)) SiO₂ layer, the 1^(st) SiO₂ layer, comprising:        a plurality of separate sensor areas;    -   (b) at least one heating element in contact with the 1^(st) SiO₂        layer and located near at least one edge of a sensor area;    -   (c) a plurality of electrical leads, each in contact with the        1^(st) SiO₂ layer, wherein 1 pair of electrical leads is at        least partly located on each of the separate sensor areas;    -   (d) a plurality of doped, metal oxide layers, wherein 1 doped,        metal oxide layer is located on each of the plurality of sensor        areas and is in contact with at least a part of the pair of        electrical leads located on the same area, wherein each doped,        metal oxide layer, comprises:        -   i. a 1^(st) metal oxide; and,        -   ii. a 2^(nd) metal oxide.

In another aspect, the present invention provides a novel chemicalsensor platform, comprising:

-   -   (a) an oxidized silicon membrane, comprising a silicon (Si)        layer and a silicon oxide (SiO₂) layer, wherein the SiO₂ layer        is located on top of the silicon layer and, comprises: a        plurality of separate sensor areas;    -   (b) at least one heating element in contact with the SiO₂ layer        and located near at least one edge of each sensor area;    -   (c) a plurality of pairs of electrical leads, each in contact        with the SiO₂ layer, wherein 1 pair of electrical leads is at        least partly located on each of the separate sensor areas;    -   (d) a plurality of doped, metal oxide layers, wherein 1 doped,        metal oxide layer is located on each of the plurality of sensor        areas and is in contact with at least a part of the pair of        electrical leads located on the same area, wherein each doped,        metal oxide layer, comprises:        -   i. a 1^(st) metal oxide; and,        -   ii. a 2^(nd) metal oxide.

The number of sensor areas in the chemical sensor platform varies.Examples include 2, 3, 4, 5, 6, 7, 8, 9, 10, or more. The number ofsensor areas determines the number of pairs of electrical leads, anddoped, metal oxide layers. The number of heating elements is independentof the number of sensor areas. One heating element can service more thanone sensor area. Examples of the number of heating elements includes 1,2, 3, 4, 5, or more.

In another aspect, the plurality is 4. In another aspect, the number ofsensor areas is 4.

In another aspect, in the chemical sensor platform there are 4 separatesensor areas, 1 heating element, 4 pairs of electrical leads, and 4doped, metal oxide layers.

In another aspect, in the chemical sensor platform there are 4 separatesensor areas, 1 Pt heating element, 4 pairs of Pt electrical leads, and4 SnO₂/TiO₂ metal oxide layers.

In another aspect, in the chemical sensor platform there are 4 separatesensor areas, 1 Pt heating element, 4 pairs of Pt electrical leads, 4SnO₂/TiO₂ metal oxide layers, and 4 Si/SiO₂ connectors.

In another aspect, in the chemical sensor platform there are 4 separatesensor areas, 1 Pt heating element, 4 pairs of Pt electrical leads, 4SnO₂/TiO₂ metal oxide layers, and 4 SiO₂/Si/SiO₂ connectors.

In another aspect, in the chemical sensor platform there are 4 separatesensor areas, 1 Pt/Ti (Ti being the 2nd material) heating element, 4pairs of Pt/Ti (Ti being the 2nd material) electrical leads, and 4 4SnO₂/TiO₂ metal oxide layers.

In another aspect, in the chemical sensor platform there are 4 separatesensor areas, 1 Pt/Ti (Ti being the 2nd material) heating element, 4pairs of Pt/Ti (Ti being the 2nd material) electrical leads, 4 SnO₂/TiO₂metal oxide layers, and 4 Si/SiO₂ connectors.

In another aspect, in the chemical sensor platform there are 4 separatesensor areas, 1 Pt/Ti (Ti being the 2nd material) heating element, 4pairs of Pt/Ti (Ti being the 2nd material) electrical leads, 4 SnO₂/TiO₂metal oxide layers, and 4 SiO₂/Si/SiO₂ connectors.

The description herein applies to both sensors and platforms, where everappropriate.

In another aspect, wherein the 1^(st) and 2^(nd) metal oxides of thedoped, metal oxide layer, are simultaneously deposited via sputtering.

In another aspect, each 1^(st) metal oxide is independently selectedfrom: SnO₂, ZnO, V₂O₅, WO₃, TiO₂, Al₂O₃, and Fe₂O₃. In another aspect,each 1^(st) metal oxide is SnO₂.

In another aspect, each 2^(nd) metal oxide is independently selectedfrom: TiO₂, Au, CuO, Cu₂O, MoO₂, MoO₃, NiO, Ni₂O₃, Pt, Pd, AgO, RuO₂,Rh₂O₃, OsO₂, OsO₄, IrO₂, and In₂O₃. In another aspect, each 2^(nd) metaloxide is In₂O₃. In another aspect, each 2^(nd) metal oxide is TiO₂.

In another aspect, each doped, metal oxide layer is independently about5, 10, 15, 20, 25, 30, 35, to 40 nm thick.

In another aspect, for each doped, metal oxide layer, the weight % ofthe 1^(st) metal oxide is from 50-99% and the weight % of the 2^(nd)metal oxide (dopant) is from 1-50%. The weight % is based on energydispersive analysis X-Ray (EDAX) of the sensing layer. Examples of theweight % of the 1^(st) metal oxide include: 50, 51, 52, 53, 54, 55, 56,57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74,75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92,93, 94, 95, 69, 97, 98, and 99. Examples of the weight % of the 2^(nd)metal oxide include: 50, 49, 48, 47, 46, 45, 44, 43, 42, 41, 40, 39, 38,37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20,19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, and 1.

In the chemical sensor (or platform), the 1^(st) SiO₂ layer is typicallypolished. The sensor area is where at least part of a pair of electricalleads is located as well as the metal oxide and dopant layers. Theheating element is not in contact with the electrical leads, the metaloxide layer, or the dopant layer but is located close enough to be ableto heat the metal oxide and dopant layers. The dopant layersubstantially if not entirely covers the exposed or top side of themetal oxide layer.

In another aspect, the oxidized silicon wafer is about 100, 150, 200,250, 300, 350, 400, 450, to 500 μm thick. In another aspect, theoxidized silicon wafer is about 200 μm thick.

In another aspect, the part of the 2nd SiO₂ layer located beneath theplurality of sensor areas (or sensor area, if only 1 is present) isabsent and a substantial portion of the corresponding silicon layer isabsent. In this aspect, part of the bottom of the wafer is absent,including all of the 2nd SiO₂ layer and some of the bottom of thesilicon layer.

In another aspect, the corresponding part of the silicon layer locatedbeneath the plurality of sensor areas (or sensor area, if only 1 ispresent) is about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70,75, 80, 85, 90, 95, to 100 μm thick. This is measured from the bottom ofthe 1^(st) SiO₂ layer to the bottom of the wafer (no 2^(nd) SiO₂ layeris present on this part of the silicon layer). In another aspect, thecorresponding part of the silicon layer located beneath plurality ofsensor areas (or sensor area, if only 1 is present) is about 50 μmthick.

In another aspect, part of the 1^(st) SiO₂ layer at the edges of theplurality of sensor areas (or sensor area, if only 1 is present) isabsent, thereby forming a discontinuous trench around the plurality ofsensor areas (or sensor area, if only 1 is present). The 1^(st) SiO₂layer that is in contact with the electrical leads remains. The absenceof the 1^(st) SiO₂ layer at the edges of the sensor area, but notincluding the 1^(st) SiO₂ layer that is in contact with the electricalleads, creates a trench that partially isolates the 1^(st) SiO₂ layer inthe sensor area from the 1^(st) SiO₂ layer outside of the sensor area.This trench can be deepened by removal of the silicon at the bottom ofthe trench. Finally, when the 2^(nd) SiO₂ under the sensor area isremoved and part of the corresponding part of the silicon layer isremoved, the trench becomes an actual opening. The remaining 1^(st) SiO₂layer in the sensor area and the corresponding silicon layer underneathare then “floating”. The floating area is called a membrane.

In another aspect, part of the 1^(st) SiO₂ layer at the edges of theplurality of sensor areas (or sensor area, if only 1 is present) andpart of the corresponding silicon layer is absent, thereby forming adiscontinuous trench around the plurality of sensor areas (or sensorarea, if only 1 is present).

In another aspect, in the chemical platform (or chemical sensor):

-   -   i. the part of the 2^(nd) SiO₂ layer located beneath the        plurality of sensor areas (or sensor area, if only 1 is present)        is absent and a substantial portion of the corresponding part of        silicon layer is absent; and,    -   ii. the part of the 1^(st) SiO₂ layer at the edges of the        plurality of sensor areas (or sensor area, if only 1 is present)        and the silicon layer directly beneath is absent, thereby        forming a discontinuous opening around the plurality of sensor        areas (or sensor area, if only 1 is present).

In another aspect, the corresponding part of the silicon layer is about5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90,95, to 100 μm thick. This is measured from the bottom of the 1st SiO₂layer to the bottom of the wafer (no 2nd SiO₂ layer is present on thispart of the silicon layer). In another example, the corresponding partof the silicon layer is about 50 μm thick.

In another aspect, the 1^(st) and 2^(nd) SiO₂ layers (in the sensor orplatform) are independently about 200, 250, 300, 350, to 400 nm thick.In another aspect, the 1^(st) and 2^(nd) SiO₂ layers are independentlyabout 300 nm thick.

In another aspect, the 1^(st) metal oxide of the plurality of doped,metal oxide layers is the same. In another aspect, the 1^(st) metaloxide of the plurality of doped, metal oxide layers is different. Inanother aspect, all of the 1^(st) metal oxide layers are the samethickness. In another aspect, all of the 1^(st) metal oxide layers areof different thicknesses.

In another aspect, the 2^(nd) metal oxide of the plurality of doped,metal oxide layers is the same. In another aspect, the 2^(nd) metaloxide of the plurality of doped, metal oxide layers is different. Inanother aspect, all of the 2^(nd) metal oxide layers are the samethickness. In another aspect, all of the 2^(nd) metal oxide layers areof different thicknesses.

In another aspect, the at least one heating element (or heating elementfor the chemical sensor), independently comprises: a 1^(st) materialselected from Pt, Au, and poly-silicon. In another aspect, the at leastone heating element, comprises: Pt.

In another aspect, the heating element is about 50, 100, 150, 200, 250,300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950 to1,000 nm thick. In another aspect, the heating element is about 300 nmthick.

In another aspect, the heating element, further comprises: a 2^(nd)material layer sandwiched between the 1^(st) SiO₂ layer and the 1^(st)material layer. In another aspect, the 2^(nd) material layer, comprises:a metal selected from Ti and Cr. In another aspect, the 2^(nd) materiallayer, comprises: Ti. In another aspect, the 2^(nd) material layer isabout 1, 2, 3, 4, 5, 6, 7, 8, 9, to 10 nm thick. In another aspect, the2^(nd) material layer is about 2 nm thick. In another aspect, the 2^(nd)material layer is about 5 nm thick.

In another aspect, the plurality of electrical leads (or electrical leadin the chemical sensor), comprise: a 1^(st) metal layer independentlyselected from Pt and Au. In another aspect, the plurality of electricalleads, comprise: Pt. In another aspect, the plurality of electricalleads are about 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550,600, 650, 700, 750, 800, 850, 900, 950 to 1,000 nm thick. In anotheraspect, the plurality of electrical leads (or lead in the chemicalsensor) are about 300 nm thick.

In another aspect, the plurality of electrical leads (or electrical leadin the chemical sensor), each further comprise: a 2^(nd) metal, layersandwiched between the 1^(st) SiO₂ layer and the 1^(st) metal layer. Inanother aspect, each 2^(nd) metal layer, comprises: a metalindependently selected from Ti and Cr. In another aspect, each 2^(nd)metal layer, comprises: Ti. In another aspect, each 2^(nd) metal layeris independently about 1, 2, 3, 4, 5, 6, 7, 8, 9, to 10 nm thick. Inanother aspect, each 2^(nd) metal layer is independently about 2 nmthick. In another aspect, each 2^(nd) metal layer is independently about5 nm thick.

In another aspect, the 2^(nd) SiO₂ layer under the sensor area is absentand the thickness of the sensor area, as measured from the top of thedopant layer to the bottom of the silicon layer (i.e., the thickness ofthe sensor membrane), is from 50, 100, 150, 200, 250, 300, 350, 400, 450to 500 μm. In another aspect, the membrane thickness is 200 μm. Inanother aspect, the membrane thickness is 100 μm.

In another aspect, the part of the 2^(nd) SiO₂ layer located beneath theplurality of sensor areas is absent and a substantial portion of thecorresponding silicon layer is absent.

In another aspect, the portions (or portion for the chemical sensor) ofthe 2^(nd) SiO₂ layer under the corresponding plurality of sensor areas(or area for the chemical sensor) is absent and the thickness of theplurality of sensor areas (or area), as measured from the top of thecorresponding dopant layers to the bottom of the corresponding siliconlayers (or layer)(i.e., the thickness of the plurality of sensormembranes (or sensor membrane)), is from 50, 100, 150, 200, 250, 300,350, 400, 450 to 500 μm. In another aspect, the thickness of theplurality of membranes (or membrane) is 200 μm. In another aspect, thethickness of the plurality of membranes (or membrane) is 100 μm. Inanother aspect, the thickness of the plurality of membranes (ormembrane) is 50 μm.

In another aspect, the present invention provides a novel method offorming a chemical sensor platform, comprising:

-   -   (a) depositing at least one heating element and a plurality of        pairs of electrical leads (e.g., 4 pairs) onto an oxidized        silicon wafer, wherein:        -   i. the oxidized silicon wafer, comprises: a silicon layer            sandwiched between a top (1^(st)) silicon oxide (SiO₂) layer            and a bottom (2^(nd)) SiO₂ layer;        -   ii. the 1^(st) SiO₂ layer, comprises: a plurality of            separate sensor areas (e.g., 4);        -   iii. the at least one heating element and plurality of pairs            of electrical leads are deposited onto the 1^(st) SiO₂            layer;        -   iv. the at least one heating element is located near at            least one edge of at least one sensor area; and,        -   v. 1 pair of electrical leads is at least partly located on            each of the separate sensor areas (e.g., 4 pairs);    -   (b) depositing a doped, metal oxide layer onto each of the        plurality of sensor areas and the 1 pair of electrical leads        located thereon, wherein the doped, metal oxide layer,        comprises:        -   i. a 1^(st) metal oxide; and,        -   ii. a 2^(nd) metal oxide; and,    -   (c) annealing the resulting platform at a sufficient temperature        and for a sufficient time to cause at least a portion of each        1^(st) metal oxide (e.g., 4 1^(st) metal oxides) to form        nanograins and at least a portion of each 2^(nd) metal oxide        (e.g., 4 2^(nd) metal oxide) to form nanoparticles.

Nanocrystals are the building blocks of nanograins/nanoparticles.Nanocrystals agglomerate into nanograins/nanoparticles (nanograins beinglarger than nanoparticles). The size of nanograins/nanoparticles vary ina range from about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65,70, 75, 80, 85, 90, 95, to 100 nm, depending on the type of material andits ability to agglomerate into bigger particles.

In another aspect, the 1^(st) and 2^(nd) metal oxides of the doped,metal oxide layer, are simultaneously deposited via sputtering.

In the method, each metal oxide layer is in contact with at least a partof 1 pair of electrical leads and part of the sensor area not covered bythe electrical leads. It is noted that neither the electrical leads northe metal oxide layer is typically in contact with the heater. Thedopant layer is substantially the same dimensions as the metal oxidelayer and substantially covers the top side of the metal oxide layer.

In another aspect, the method, further comprises:

-   -   (d) etching away part of the 1^(st) SiO₂ layer at the edges of        the plurality of sensor areas and part of the corresponding        silicon layer to form a discontinuous trench around the        plurality of sensor areas.

In another aspect, the method, further comprises:

-   -   (e) etching away the part of the 2^(nd) SiO₂ layer located        beneath the plurality of sensor areas and part of the        corresponding silicon layer, wherein enough of the silicon layer        is removed to convert the discontinuous trench into a        discontinuous opening in the silicon wafer.

In another aspect, plurality is 4.

In another aspect, etching (f) is completed prior to annealing (e).

In another aspect, the method, further comprises:

-   -   (f) prior to depositing (a), applying a 1^(st) photomask to the        1^(st) SiO₂ layer; and,    -   (g) after depositing (a), removing the 1^(st) photomask.

In another aspect, the method, further comprises:

-   -   (h) prior to etching (e), applying a 2^(nd) photomask to the        1^(st) SiO₂ layer; and,    -   (i) after etching (e), removing the 2^(nd) photomask.

In another aspect, the method, further comprises:

-   -   (j) prior to depositing (b), applying a 3^(rd) photomask to the        1^(st) SiO₂ layer; and,    -   (k) after depositing (c), removing the 3^(rd) photomask.

In another aspect, the method, further comprises:

-   -   (l) prior to etching (f), applying a 4^(th) photomask to the        2^(nd) SiO₂ layer; and,    -   (m) after etching (f), removing the 4^(th) photomask.

In another aspect, the method, further comprises:

-   -   (n) prior to depositing (a), depositing an adhesive metal layer

The adhesive metal layer is the 2^(nd) material layer sandwiched betweenthe 1^(st) SiO₂ layer and the 1^(st) material layer.

In another aspect, the present invention provides a novel, integratedchemical sensor that can be used in the following applications:

-   -   a. Sensor components for confined space gas monitors, leak        detectors and analytical instruments. The present sensor can        replace traditional metal oxide sensors in their standard        applications for gas detection. The application determines the        type of sampling system (active or passive).    -   b. Analytical instruments for analysis of chemical compounds in        natural gas and oil    -   c. Detectors of explosives    -   d. Breathalyser for analysis of THC and other drugs and their        metabolites in the human breath.    -   e. Alcohol monitors inside vehicles for prevention of drunk        driving.    -   f. Sensors utilized in cooking processes for prevention of        overcooking and burning.    -   g. Built-in sensors for cell phones    -   h. Built-in sensors for microphones.    -   i. Built-in sensors for food freshness and safety monitoring for        refrigerators.

EXAMPLES

The following examples are meant to illustrate, not limit, the presentinvention.

Example 1

A general description of the novel manufacturing process for makingsensors and the highly-integrated sensors of the present invention is asfollows:

Starting Substrate:

An oxidized silicon wafer is used as the substrate for the sensors andplatforms of the present invention. A platform of the present inventionis a unit that comprises multiple sensors and pairs of electrical leadsand at least one heating element. The present manufacturing processallows for numerous platforms to be formed simultaneously on oneoxidized silicon wafer. As will be described below, each platform maycontain only one type of sensor (e.g., all sensors have the same doped,metal oxide layers) or each sensor on the platform may have a variety ofdifferent sensor types (e.g., 2, 3, 4, or more).

A typical wafer is 4″ in diameter, but a larger wafer (e.g., 6″ indiameter) can be used in order to increase the number of devices permanufacturing cycle and decrease the manufacturing cost per device. Theorientation of the wafer can be 100. One side (the “top” side) of thewafer is typically polished. An example of the thickness of the wafer is200 μm. Other examples of the thickness of the wafer include from about100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230,240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370,380, 390, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, to 500 μmthick. The thinner the wafer (e.g., 200 μm, 150 μm, or 100 μm), thelower the power consumption of the resulting device.

A 1^(st) layer of oxidized silicon (SiO₂) is present on the top of thewafer. A 2^(nd) layer of oxidized silicon (SiO₂) is present on thebottom of the wafer. Since the wafer is polished, it is the 1^(st) layerof SiO₂ that is polished. An example of the thickness of the oxidelayers is about 300 nm. Other examples of the thickness of the SiO₂layers include from about 200, 210, 220, 230, 240, 250, 260, 270, 280,290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390 to 400 nm thick.

Applying Photomask #1

Each sensor has to be thermally activated in order to act as aconductometric sensor. Thus, a microheater (heating element) has to bedeposited for each sensor or on each platform. Also, interdigitatedmetal terminals (electrical leads) have to be deposited for monitoringof signals from each of the sensor areas (change in conductance causedby a chemical reaction). Patterns for both the microheater andterminals, for the numerous platforms that can be simultaneously made,can be deposited on the wafer using photolithography, as follows.

A photoresist (Photomask #1) is spin coated on the front side (i.e.,polished side) of the wafer. The photoresist is chosen based on itsdesired thickness (thick enough to allow for deposition of the heaterand metal leads, but not too thick to make it difficult to use). Forexample, photoresist 1827, a positive photoresist that is expected toproduce a 2.7 μm layer @ 4000 rpm spin, can be used. It is applied byspinning the wafer for 0.2 sec@ 500 rpm while the photoresist isapplied, and then for 10 sec @ 4000 rpm. Other examples of the thicknessof the photoresist to be applied include from about 2, 3, 4, 5, 6, 7, 8,9, to 10 μm.

After spin coating, the wafer is “soft baked” by heating at (e.g., 90°C. in air for about 70-75 sec). The “soft baked” wafer is then exposedto UV light appropriate for the photoresist (e.g., 325 W for about 22seconds), followed by a toluene bath (60 sec), blow drying with N₂, andanother soft baking (e.g., 90° C. in air for about 15 sec).

In order to remove the unwanted portions of the photoresist, it must bedeveloped, rinsed, and dried. For example, the photoresist can bedeveloped by contacting with a developing agent (e.g., MF-24A for 90sec), followed by a Quick Dump Rinse (QDR) with dionized water. Finally,the rinsed wafer can then dried via a Spin Rinse Dryer (SRD). Forexample, it can be spun for 30 sec @ 500 rpm with a deionized waterspray, then 3 min @ 2000 rpm under a N₂ gas flow, and finally for 3 min@ 4000 rpm in air to dry.

The quality of photolithography can be verified by optical microscopy tomatch the desirable percent of defects, desired geometry, and thephotoresist free area. If the photoresist is not fully removed, thewafer can be placed in the developer bath (e.g., contacted with MF-24A)for an additional 10-15 sec, then cleaned via the QDR and SRD methodsand then checked again by optical microscopy.

Heater and Electrical Lead Deposition

With the photomask applied, the heater and electrical leads are thenformed. One way to achieve thin and uniform layers of heater and/orelectrical leads (as well as the other layers/components of the sensor)is via sputtering. Other techniques including atomic layer deposition,chemical vapor deposition, and thermal evaporation can also be used forthe heater and electrical leads, as well as for the otherlayers/components of the sensor.

Optionally an adhesive layer (e.g., Ti or Cr) can be deposited first,followed by the desired material for heater (e.g., Pt, Au, orpoly-silicon) and/or electrical lead (Pt and Au). The adhesive layer canbe deposited by sputtering using an appropriate target (e.g., a Tiwafer).

An example of the thickness of the optional adhesive layer is about 2nm. Another example is about 5 nm. Other examples of the thickness ofthe optional adhesive layer include from about 1, 2, 3, 4, to 5 nm. Anexample of the thickness of the electrical lead is about 300 nm. Otherexamples of the thickness of the electrical lead include from about 100,150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800,850, 900, 950, to 1000 nm. An example of the thickness of the heater isabout 300 nm. Other examples of the thickness of the heater include fromabout 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700,750, 800, 850, 900, 950, to 1000 nm.

It can also be desirable to deposit alignment marks to allow foradditional photomasks to be aligned. These marks are not limited byshape (e.g., X's, crosses, boxes, etc.) and need only be large enough tobe seen when aligning another photomask.

Removing Photomask #1:

Once the electrical leads and heater have been applied, Photomask #1 isremoved to allow for the deposition of the metal oxide layer. Photomask#1 can be removed using standard technology. For example, the modifiedwafer can be placed in an acetone bath for 2 hours at 60-70° C. Thewafer can then be placed in a sonicated bath for 5-10 min to remove theremaining metal coated photoresist. An optical microscope can be usedfor quality control of metal deposition. The thickness of the depositedmetal layer can then be verified using a contact profilometer.

FIG. 1 shows an example of a part of a wafer (green) to which a heatingelement (white area surrounding the green “T” in the middle) andelectrical leads (interdigitated terminals)(remaining white area) havebeen deposited and Photomask #1 has been removed. The heater in thisfigure separates 4 sensor areas.

In another aspect, the heater and electrical leads are differentmaterials, e.g., a poly-silicon heater and Pt leads. If differentmaterials are chosen, then an additional photomasking step will benecessary. Either the heater or electrical leads are applied while thelocation on the wafer for the other is protected. Photomask #1 is thenremoved and #1A is applied to allow for the other of the heater orelectrical leads to be applied. Photomask #1A is then removed. Thestructure shown in FIG. 1 could also be an example wherein the heaterand electrical leads are different materials.

Applying Photomask #2:

In order to deposit the metal oxide, the heater (and optionally otherparts of the wafer) needs to be protected. Photomask #2 can be appliedsimilarly to the Photomask #1. It is useful to be able to alignPhotomask #2 with Photomask #1. For example, Photomask #1 can havecrosses on both sides of the mask and Photomask #2 can have squares.

As with Photomask #1, the quality of photolithography should be verifiedby optical microscopy to match the geometry and desirable percent ofdefects. If the developed photoresist is not fully removed, the wafershould be placed again in the developer (e.g., contacted with MF-24A foran additional 10-15 sec), then cleaned via the described QDR and SRDmethods and checked again by optical microscopy.

Trenching:

RIE etching: One way to decrease the power required to heat the metaloxide is to substantially isolate the sensor area from the surroundingwafer. This can be achieved first by RIE etching of the SiO₂ layer toform a discontinuous trench around the sensor area (see the black linesshown in FIG. 2). The trench is discontinuous as the SiO₂ under theelectrical leads and heater is not removed. The etched area can beinspected under an optical microscope. The process is repeated ifnecessary to remove the SiO₂ completely in the desired areas.

One way to separate the platforms from the wafer is via etching. Forexample, the SiO₂ from an outline around each platform can be etched asa part of the RIE etching process. This begins the process of creating624 separate sensor platforms from the original wafer.

Before proceeding to etch part of the silicon layer, it is useful tomeasure the thickness of the photoresist. If the photoresist thicknessis less than 2˜μm after RIE, then the photo resist can be removed andreapplied. A thicker photoresist (e.g., SPR 220-7 (provides ˜7 μm layerat 4000 rpm)) can be chosen to protect the features from the DRIEetching that is to follow. It can be beneficial to trench with DRIEimmediately after removing the SiO₂ layer to avoid new oxide formation.

DRIE etching: Prior to DRIE etching, the silicon wafer is protected byattaching a support silicon wafer (e.g., a 500 μm support silicon wafer)to the back side of the silicon wafer. The support wafer protects theprocessed wafer from being broken in the DRIE chamber. The silicon onthe front side of the wafer that was exposed during RIE etching is thenetched via DRIE. The etched area is inspected under optical microscope.The process is repeated if necessary to finish removing the unwantedsilicon.

If the platforms are to be separated by etching, this process can becontinued with the DRIE. For example, the silicon in the outline aroundeach platform formed by the above SiO₂ etching can also be etched tocontinue the process of creating 624 separate sensor platforms from theoriginal wafer.

Removing Photomask #2:

Photomask #2 can then be removed using the lift-off process describedfor Photomask #1.

Sensing Element Fabrication:

A third photomask (Photomask #3) can be applied similarly to thePhotomask #1 and aligned similarly to Photomask #2 (e.g., squares onPhotomask #3 can be aligned with crosses from Photomask #1). As withPhotomask #1, the quality of photolithography should be verified byoptical microscopy to match the geometry and desirable percent ofdefects. If the developed photoresist is not fully removed, the wafershould be placed again in developer (e.g., contacted with MF-24A for anadditional 10-15 sec), then cleaned via the described QDR and SRDmethods and checked again by optical microscopy.

With Photomask #3 in place, a doped metal oxide can be deposited. Auseful method of depositing the doped, metal oxide layer is viaco-sputtering (both metal oxide and dopant are sputteredsimultaneously). This method allows one to precisely control thethickness of the layer being deposited and also provides a consistentand repeatable process for forming the metal oxide layer (as opposed tomethods such as using a sol-gel paste). The nanograins/nanoparticles areformed during the annealing process (see below).

One example of a 1^(st) metal oxide is SnO₂. Other examples include ZnO,V₂O₅, WO₃, TiO₂. Al₂O₃, In₂O₃ and Fe₂O₃.

One example of a 2^(nd) metal oxide (dopant) is TiO₂ (the TiO₂ can besputtered as TiO₂ or as Ti and then oxidized via annealing). Otherexamples include TiO₂, Au, CuO, Cu₂O, MoO₂, MoO₃, NiO, Ni₂O₃, Pt, Pd,AgO, RuO₂, Rh₂O₃, OsO₂, OsO₄, IrO₂, and In₂O₃. The dopant if sputteredas a non-oxide metal (e.g., Ti, Au, Cu, Mo, Ni, Pt, Pd, Ag, Ru, Rh, Os,Ir, and In) is typically, with the exception of Au, Pt, and Pd, oxidizedduring annealing (see below). This oxidation process oxidizes part ofthe dopant (typically the area exposed to oxygen), but does notnecessarily oxidize all of the dopant. For example, some of the dopantinside may still be in an unoxidized state. Au, Pt, and Pd, while notforming oxides during annealing, do form quasi-oxide states on theirsurfaces (e.g., quasi-stable clusters such as Pd_(n)O_(m) andPt_(n)O_(m)).

An example of the thickness of the doped, metal oxide layer is about 36nm. Other examples of the thickness of the doped, metal oxide layerinclude from about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, to 50 nm dependingon desired the oxide parameters. The thickness of the doped, metal oxidelayer can be measured by a contact profilometer.

The different combinations of metal oxide and dopant provide differentsensitivities to different gases.

With the metal oxide layer formed, Photomask #3 can be removed asdescribed for Photomask #1.

Different Sensing Elements:

If the sensor platform contains sensors with different doped metaloxides and/or (i.e., different sensors), then the process of applying aphotomask, developing it, forming the doped, metal oxide layers, andremoving the photomask is repeated for as many times as necessary. Forexample, if each platform contains 4 different sensors separated by aheater (similar to the structure shown in FIG. 2), then process ofapplying the doped, metal oxide layers will be repeated three additiontimes (Photomasks 3A, 3B, and 3C will be used) to provide a sensor with4 distinct doped, metal-oxide sensing areas.

“Floating” Sensor Formation:

After all the sensing elements are deposited and doped, the bottom ofthe wafer is then etched to remove the SiO₂ layer under the sensor area(where the doped, metal oxide layers are located) as well as asubstantial part of the bottom of the silicon layer. For example, if thesilicon layer is 200 μm thick, then about 150 μm can be removed, therebyleaving only about 50 μm of silicon under the sensor and creating a“floating” sensor by removing enough silicon to convert the trenchespreviously formed into openings (i.e. holes). Removal of the silicon andforming the openings reduces the energy needed to power the sensorplatform (and resulting integrated sensor).

The features on the top of the wafer should be protected with a layer ofphotoresist (e.g., photoresist 1813 can be applied). The bottom of thewafer can now be modified.

A photoresist (Photomask #4)(e.g., photoresist SPR 220-7) can be appliedonto the backside (unmodified side) of the wafer similarly to previousphotomasks. After exposure to UV light and developed, the photomask iscomplete and etching can begin. Photomask #4 is typically thicker thanPhotomasks #1-3 because of the amount of silicon being removed duringthe DRIE process. As a result, more time is usually required to allowthe thicker photoresist layer, e.g., 4-10 μm to cure.

The bottom SiO₂ layer can be removed via RIE. The etched area is theninspected under optical microscope. The process is repeated if necessaryto remove the oxide completely.

If the platforms are to be separated by etching, this process can becontinued with the RIE on the bottom of the wafer. For example, the SiO₂on the bottom of the wafer that is under the outline formed above aroundeach platform can also be etched to continue the process of creating 624separate sensor platforms from the original wafer.

Next part of the silicon layer is removed via DRIE. It can be beneficialto start DRIE right after removing the oxide to avoid new oxideformation over time.

Due to the harsh conditions of the DRIE process, the silicon wafer canfirst be protected by attaching a support silicon wafer (e.g., a 500 μmsupport silicon wafer) to the top side of the silicon wafer. The supportwafer protects the processed wafer from being broken in the DRIEchamber. Once the top of the silicon has been protected, silicon fromthe bottom can then be etched, e.g., ˜150 μm, via DRIE. The etched areais inspected under optical microscope. The process is repeated ifnecessary to complete removal of the desired amount of silicon.

If the platforms are to be separated by etching, this process can becompleted with the DRIE on the bottom of the wafer. For example, thesilicon in the outline around each platform formed by SiO₂ etching onthe bottom can also be etched to complete the process of cutting all theway through the wafer and of creating 624 separate sensor platforms fromthe original wafer.

With the removal of SiO₂ and silicon complete, Photomask #4 can beremoved as described for Photomask #1.

The amount of silicon removed from under the sensing area depends on thedesired thickness of the “floating” sensor. One example of the thicknessof silicon under the sensing area is 50 μm. Other examples include fromabout 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37,38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55,56, 57, 58, 59, to 60 μm.

An example of the “floating” sensor of the present invention is shown inFIG. 3. The dark grey lines on either side of the Pt leads representsdiscontinuous openings (holes) in the wafer that were created by theRIE/DRIE trenching and then RIE/DRIE removal of the bottom side of thewafer.

The depth of the DRIE is typically such that it cuts apart the platformswithout the need for mechanical cutting. Optionally, one can reduce thedepth of the DRIE etching and then cut the platforms apart mechanically.

Sensor Packaging

Bonding conditions: Gold wire diameter 25 μm, substrate temperature 130°C., tail 2, loop 2. Kulicke & Soffa Wire bonder Model 4500.

-   -   a. First bond (ball bond): force 2, power 2, time 2.    -   b. Second bond (wedge bond): force 3, power 2.5, time 2.

With the sensor platform complete, it is then connected to a package inorder to be incorporated into an integrated chemical sensor. The surfaceof each sensor's platform has to be leveled with the package pin holdersin order to improve the bonding contacts and prevent the wire frombreaking or being stuck inside the soldering capillary. The sensors cupare attached in order to complete the integrated sensor package.

Sensor Annealing

In order for nanoparticles and nanograins to form in the doped, metaloxide layers, and if necessary to oxidize the 2^(nd) metal oxide (ifdeposited as non-oxide, e.g., Ti or In), the sensor is annealed in thepresence of oxygen (e.g., air or synthetic air). The annealing can beconducted prior to packaging. However, a benefit of annealing afterpackaging is that the platform need not be touched post packaging.

Annealing temperature: 200-900° C., including 200, 250, 300, 350, 400,450, 500, 550, 600, 650, 700, 750, 800, 850, to 900° C.

Annealing time: 1-40 hours, including 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29,30, 31, 32, 33, 34, 35, 36, 37, 38, 39, to 40 hours.

Annealing atmosphere: an oxygen-containing gas, including air andsynthetic air.

The temperature chosen is dependent up on the components of the sensorand its desired use. The size of the grains/particles and theirmorphology can be observed via SEM (scanning electron microscope), AFM(atomic force microscope), and/or XRD (x-ray diffraction).

During the annealing process metal oxide grains (nanograins) of the1^(st) metal oxide are formed from the amorphous, sputtered structure.The size of the grains (e.g., 5-20 nm) impacts the sensitivity of thesensor.

During the annealing process the dopant (2^(nd) metal oxide), if notalready oxidized (e.g., Ti sputtered as opposed to TiO₂), oxides anddopant nanoparticles are formed. The formation of dopant nanoparticlescauses the continuous amorphous layer (e.g., formed via sputtering) iscrystallized to form a polycrystalline structure.

In one example, the annealing temperature for SnO₂ is 700° C., for 4hours.

In one example, the annealing temperature for ZnO is 700° C., for 4hours.

Example 2

Examples of the weight percentages of doped, metal oxide layers of thepresent invention are shown in Table 1 below.

TABLE 1 Doped Metal Oxide Layer 1^(st) Metal Oxide 2^(nd) Metal OxideExample (weight %) (weight %) 1. 50 50 2. 55 45 3. 60 40 4. 65 35 5. 7030 6. 75 25 7. 80 20 8. 85 15 9. 90 10 10. 95 5 11. 96 4 12. 97 3 13. 982 14. 99 1

Example 3

FIG. 3 shows a floating sensor or membrane of the present inventionhaving Pt electrodes, a Pt heater, SiO₂/Si/SiO₂ connectors, and aSnO₂—TiO₂ doped, metal oxide layer (dual oxide in the figure). Examplesof the weight percentages of the SnO₂—TiO₂ layer shown in FIG. 3 areshown in Table 2 below.

TABLE 2 SnO₂—TiO₂ Layers SnO₂ TiO₂ Example (weight %) (weight %) 1. 5050 2. 55 45 3. 60 40 4. 65 35 5. 70 30 6. 75 25 7. 80 20 8. 85 15 9. 9010 10. 95 5 11. 96 4 12. 97 3 13. 98 2 14. 99 1

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise that as specifically described herein.

We claim:
 1. A chemical sensor platform, comprising: (a) a floating,oxidized silicon membrane, comprising: a silicon (Si) layer and asilicon oxide (SiO₂) layer, wherein the SiO₂ layer is located on top ofthe silicon layer and, comprises: a plurality of separate sensor areas;(b) at least one heating element in contact with the SiO₂ layer andlocated near at least one edge of each sensor area; (c) a plurality ofelectrical leads, each in contact with the SiO₂ layer, wherein one pairof electrical leads is at least partly located on each of the separatesensor areas; (d) a plurality of doped, metal oxide layers, wherein onedoped, metal oxide layer is located on each of the plurality of sensorareas and is in contact with at least a part of the pair of electricalleads located on the same area, wherein each doped, metal oxide layer,comprises: (i.) a first metal oxide; and, (ii.) a second metal oxide;and, (e) a plurality of Si/SiO₂ connectors that are substantiallyisolated from the floating membrane.
 2. The chemical sensor platform ofclaim 1, wherein there are four Si/SiO₂ connectors.
 3. The chemicalsensor platform of claim 1, wherein each first metal oxide is tin oxide(SnO₂).
 4. The chemical sensor platform of claim 1, wherein each secondmetal oxide is titanium oxide (TiO₂).
 5. The chemical sensor platform ofclaim 1, wherein the doped, metal oxide layer is about five to fortynanometers thick.
 6. The chemical sensor platform of claim 1, whereinthe weight percentage of each first metal oxide is from fifty to ninetynine percent and the weight percentage of the second metal oxide is fromone to fifty percent.
 7. The chemical sensor platform of claim 1,wherein there are four separate sensor areas, one heating element, fourpairs of electrical leads, and four doped, metal oxide layers.
 8. Thechemical sensor platform of claim 1, wherein there are four separatesensor areas, one platinum (Pt) heating element, four pairs of Ptelectrical leads, and four SnO₂/TiO₂ metal oxide layers.
 9. The chemicalsensor platform of claim 1, wherein there are four separate sensorareas, one Pt heating element, four pairs of Pt electrical leads, fourSnO₂/TiO₂ metal oxide layers, and four SiO₂/Si/SiO₂ connectors.
 10. Thechemical sensor platform of claim 1, wherein there are four separatesensor areas, one Pt/titanium (Ti) heating element, four pairs of Pt/Tielectrical leads, four SnO₂/TiO₂ metal oxide layers, and fourSiO₂/Si/SiO₂ connectors.
 11. The chemical sensor platform of claim 1,wherein each first metal oxide is SnO₂ and each second metal oxide isTiO₂.
 12. The chemical sensor platform of claim 1, wherein there is oneheating element.
 13. The chemical sensor platform of claim 1, whereinthere is one Pt heating element.
 14. The chemical sensor platform ofclaim 1, wherein there is one Pt/Ti heating element.
 15. The chemicalsensor platform of claim 1, wherein each first metal oxide is SnO₂, eachsecond metal oxide is TiO₂, and there is one heating element.
 16. Thechemical sensor platform of claim 1, wherein each first metal oxide isSnO₂, each second metal oxide is TiO₂, and there is one Pt heatingelement.
 17. The chemical sensor platform of claim 1, wherein each firstmetal oxide is SnO₂, each second metal oxide is TiO₂, and there is onePt/Ti heating element.
 18. The chemical sensor platform of claim 1,wherein there are four separate sensor areas, one Pt heating element,four pairs of Pt electrical leads, four SnO₂/TiO₂ metal oxide layers,and four Si/SiO₂ connectors.
 19. The chemical sensor platform of claim1, wherein there are four separate sensor areas, one Pt/Ti heatingelement, four pairs of Pt electrical leads, four SnO₂/TiO₂ metal oxidelayers, and four Si/SiO₂ connectors.